Implantable leads represent the electrical link between an implantable medical device (often referred to simply as “IMD”) and a subject's cardiac or other tissue, which is to be sensed or stimulated. An implantable lead may include a single or multiple conductors that are connected to an electrode or an electrode assembly at a lead intermediate portion or a lead distal end portion. A connector is included at a lead proximal end portion to form an electrical connection (via the conductor(s)) between the electrode or electrode assembly and the IMD.
Over the years, a large number of different mechanisms and methods for interconnecting conductors and electrodes have been proposed. It is desirable that such connections between the conductor and the electrode provide a highly reliable electrical connection, with good mechanical properties including high tensile strength. It is also desirable that such connections allow for the lead assembly itself to retain a high degree of tensile strength through the area of the electrode. This is because cardiac (and other) leads undergo considerable stresses due to repetitive flexing caused by, for example, the motion of a beating heart and forces applied to the lead during an implantation, repositioning, or lead extraction procedure.
Typically, conductors in commercially marketed pacing and defibrillation leads have taken the form of single or multi-filar wire coils. Recently, there has been a high level of interest in designing leads having lead bodies with a reduced size (i.e., lead body diameter) or additional electrodes. Lead body size can be reduced by employing stranded wire conductors such as cables, in lieu of coiled wire conductors. However, such stranded wire conductors present new challenges not faced by the use of coiled wire conductors. As one example, it has been a great challenge to electrically and reliably connect a small multi-strand conductor cable (often times having a cable outer diameter on the order of thousandths of an inch) to a ring electrode or a multi-filar shock coil electrode. Being of such small size, the connection is a very difficult one to make and fragile, if made incorrectly.
With respect to single or multi-filar wire coiled conductors, when such conductors are used to electrically connect a distal (tip) electrode to the IMD, portions of the distal electrode typically are polymer bonded (e.g., via an adhesive) to provide additional axial strength to the electrode. However, even with such additional polymer-provided strength, the distal electrode/lead body connection may still fall short of the axial strength necessary to resist permanent deformation or in order to pass industry standards (e.g., CEN/CENELEC 45502-2-1, §23.3).